在什么情况下原子属性有用？ [英] Under what circumstances are atomic properties useful?

问题描述

Objective-C属性默认为 atomic ，这可确保访问者是原子的，但不能确保整体线程安全性（根据这个问题）。我的问题是，在大多数并发场景中，原子属性不是多余的吗？例如：

Objective-C properties default to atomic, which ensures that accessors are atomic but doesn't ensure overall thread-safety (as per this question). My question is, aren't atomic properties redundant in most concurrency scenarios? For example:

场景1：可变属性

@interface ScaryMutableObject : NSObject {}

@property (atomic, readwrite) NSMutableArray *stuff;

@end

void doStuffWith(ScaryMutableObject *obj) {
    [_someLock lock];
    [obj.stuff addObject:something]; //the atomic getter is completely redundant and could hurt performance
    [_someLock unlock];
}

//or, alternatively
void doStuffWith(ScaryMutableObject *obj) {
    NSMutableArray *cachedStuff = obj.stuff; //the atomic getter isn't redundant
    [_someLock lock];
    [cachedStuff addObject:something]; //but is this any more performant than using a nonatomic accessor within the lock?
    [_someLock unlock];   
}

场景2：不可变属性

我在想，在处理不可变对象时，原子属性可能对避免锁有用，但由于不可变对象可以指向Objective-C中的可变对象，所以这并不是很多帮助：

I was thinking that maybe atomic properties would be useful for avoiding locks when working with immutable objects, but since immutable objects can point to mutable objects in Objective-C, this isn't really much help:

@interface SlightlySaferObject : NSObject {}

@property (atomic, readwrite) NSArray *stuff;

@end

void doStuffWith(SlightlySaferObject *obj) {
    [[obj.stuff objectAtIndex:0] mutateLikeCrazy];//not at all thread-safe without a lock
}

我能想到的唯一可以安全使用原子的场景没有锁的访问器（因此值得使用原子属性）是：

The only scenarios I can think of where it's safe to use atomic accessors without a lock (and therefore worth using atomic properties at all) are:

1. 使用
   primitives的属性;


2. 使用
   保证不可变的属性而不是
   指向可变对象（例如
   和 NSString 或$ code>
   不可变对象的NSArray 。

1. Working with properties that are primitives;
2. Working with properties that are guaranteed to be immutable and not to point to mutable objects (such as an NSString or an NSArray of immutable objects).

我错过了什么吗？有没有其他充分理由使用原子属性？

Am I missing something? Are there any other good reasons to use atomic properties?

推荐答案

你没有遗漏任何东西; atomic 的用处主要限于您需要从多个线程访问或设置特定值的情况，其中该值也是整数。

You aren't missing anything; atomic's usefulness is largely limited only to situations where you need to access or set a particular value from multiple threads where that value is also integral.

超出单个值， atomic 不能用于线程安全目的。

Beyond a single value, atomic cannot be used for thread safety purposes.

我在不久前发布的博客。

这个问题也是一个[非常好的]复制品原子和非原子属性之间有什么区别？

This question is also a [very well posed] duplicate of What's the difference between the atomic and nonatomic attributes?

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